Standard solid-phase immunoassays with monoclonal antibodies involve the formation of a complex between an antibody adsorbed on a solid phase (capture antibody), an antigen, and an antibody to another epitope of the antigen conjugated with an enzyme or a detectable label (tracer antibody). Thus, a sandwich is formed: solid phase-capture antibody-antigen-tracer antibody. In the sandwich, the activity of the antibody-conjugated enzyme (or the amount of the detectable label) is proportional to the antigen concentration in the incubation medium. One sandwich assay is the double antigen bridging immunoassay whereby capture and tracer antibodies bind to different epitopes of the antigen. Hoesel, W., et al., in J. Immunol. Methods 294 (2004) 101-110, report an anti-EPO double antigen bridging assay whereby a mixture of immobilized rhEPO coupled to amino groups and to carbohydrate groups was used. Immunoassays such as the double antigen bridging ELISA are common assay types in the investigation of an immunogenic answer of a patient to an antibody drug. Mire-Sluis, A. R., et al., in J. Immunol. Methods 289 (2004) 1-16, summarize the recommendations for the design and optimization of immunoassays using detection of host antibodies against biotechnology products. According to Mire-Sluis et al., the well-known anti-drug antibody assay formats show considerable disadvantages. Anti-drug antibody assays are mentioned, for example, in WO 2005/045058 and WO 90/006515. Anti-idiotypic antibody assays are mentioned, for example, in U.S. Pat. No. 5,219,730, WO 87/002778, EP 0 139 389, and EP 0 170 302. Wadhwa, M., et al., in J. Immunol. Methods 278 (2003) 1-17, report strategies for the detection, measurement and characterization of unwanted antibodies induced by therapeutic biologicals. An immunological assay for the immunological determination of an antibody against a drug antibody in a sample using a double antigen bridging immunoassay is described in PCT/EP2007/001935. An immunological assay for the determination of human antibodies in monkeys is described in WO 2006/066912. In the art (e.g. US 2003/0068664) also assay systems are known, which allow for the detection of active therapeutic antibodies. Such systems require the binding of the antigen to a solid phase, binding of the therapeutic antibody to this bound antigen and detection of the therapeutic antibody bound via the antigen to the solid phase.